Defrost control for refrigeration systems



June 20, 1961 w. L. MOGRATH DEFROST CONTROL FOR REFRIGERATION SYSTEMS 2Sheets-Sheet 1 Filed Feb 1. 1957 IO 20 30 4O 5O 6O 7O OUTSIDETEMPERATURE F INVENTOR. WILLIAM L. MCGRATH.

w w w w m o w. 200

3 .rzam 40:-

ATTORNEY.

June 20, 1961 w. 1.. MCGRATH 2,988,897 DEFROST CONTROL FOR REFRIGERATIONSYSTEMS Filed Feb. 1, 1957 2 Sheets-Sheet 2 F l G. 4

F l G. 5

INVENTOR. WI LLlAM L. McGRATH.

BY WJM ATTORNEY.

United States Patent 2,9883% DEFROST CONTROL 0R REFRIGERATION SYSTEMSWilliam L. McGrath, Syracuse, N.Y., assignor to Carrier Corporation,Syracuse, N.Y., a corporation of Delaware Filed Feb. 1, 1957, Ser. No.637,786 4 Claims. (Cl. 62-156) This invention relates to conditionresponsive control devices, more particularly to a control specificallyadapted to regulate the defrost cycle of a refrigeration system, whensaid system is employed in ambient surroundings of greatly varyingtemperature.

A number of instances arise wherein it is desired to regulate theoperation of a mechanism in response to thermal or pressure conditions.Thus, with refrigeration systems, in addition to the need for regulatingthe operation of the system to attain desired temperature conditions,there specifically arises a problem in conjunction with the defrostingof the evaporator coils of the system. The conventional compressionrefrigeration cycle circulates a refrigerant through a closed system inwhich the refrigerant in a gaseous state is first compressed, then fedthrough a condenser coil where the gaseous refrigerant is liquefied, andin the process gives off heat. Thereafter, the liquid refrigerant ispassed through some expansion member where it is converted to a gas andthen further expanded in an evaporator coil, absorbing heat in theprocess. From the evaporator coil the gaseous refrigerant is returned tothe compressor for recirculation through the system. The surface of theevaporator coil tends to accumulate frost thereon due to the fact thatthe surface temperature drops below 32 Fahrenheit, causing any moisturecondensed out of the air flowing over the coil to freeze on the surfaceof the evaporator coil. The build up of frost or ice on the evaporatorcoil, acts as an insulator decreasing the rate of heat transfer throughthe coil and substantially minimizing the efficiency of therefrigeration cycle and may eventually render it ineffective. As isapparent, it is thus desirable to provide some means for preventingfrost accumulation.

Defrosting of the evaporator coil may be accomplished in a variety ofways, as by stopping of the refrigeration cycle, and either blowing warmair over the evaporator coil, or discharging the relatively warmcondenser gas into the evaporator coil. Problems are engendered inproviding a control which will initiate any of the available defrostingoperations at the proper time.

(Where the refrigeration system is employed as a heat pump, that is bypositioning of the evaporator coil to permit the outside air to passthereover, at the same time removing heat from this outside air, andthen circulating theheated refrigerant through to an inside positionedcondenser where the heat is given off, the problem of obtaining asuitable control for initiating a defrost cycle becomes even morepronounced due to the variations in ambient temperature under which therefrigeration system is employed.

Conventional control devices are not very satisfactory for controllingmany systems, particularly heat pumps, which are subject to a widevariation of ambient conditions, since they generally function toinitiate the defrost cycle at a fixed suction temperature.

Simple thermostats or pressure switches have been used which cut out orinitiate the defrost cycle on a drop to a predetermined evaporatorsuction temperature and then cut back in when the defrost cycle iscompleted as the result of reaching a suction temperature in excess of32. Other defrosting methods include devices which respond to the staticpressure drop in the air stream, or respond to the velocity of the airpassing through the coil. These types are diflicult to keep inadjustment since they are exposed to cold temperatures (which may neverrise above 32 F.), and the temperature or pressure responsive devicesare not adequate because of the wide range of evaporator coiltemperatures normally encountered even in the absence of frosting.

Where heat pumps are located in temperate climate zones, the evaporatorcoil is exposed to ambient temperatures ranging from 20 F. to 100 F. Aconventional control set to initiate the defrost cycle at any fixedtemperature is obviously inadequate since defrosting may not benecessary at the control temperature. Thus, with ambient temperaturesbelow freezing there may possibly be deleterious frosting of theevaporator coil only upon a drop in temperature of the evaporator coilto a point substantially below that of the ambient surroundings, Whereasat ambient temperatures slightly above freezing, a much higherevaporator temperature indicates defrosting is necessary.

It is with the above problems in mind that the present means have beenevolved, means which permit initiation of a process in response to agiven temperature differential between two points and termination of theprocess in response to one of such temperature measurements. Whereregulation is desired of the defrosting operation of a heat pump, thecontrolling temperatures are those of the outside air and the evaporatorcoil, or between the saturated refrigerant pressure corresponding to theoutside air and the evaporator pressure which is equivalent. In suchcase defrosting is initiated as a result of a temperature differencebetween the outside air and evaporator coil, and is terminated as afunction of the suction temperature alone. Above a given outsidetemperature, defrosting is dependent solely on evaporator suction.

It is thus a primary object of this invention to provide an improvedtemperature or pressure responsive control device.

A further object of this invention is to provide an improved controldevice for use with refrigeration systems.

Another object of this invention is to provide a control for initiatingand terminating the defrost of the evaporator coils of a refrigerationsystem.

It is also a specific object of this invention to provide improvedcontrol means for regulating the defrost cycle of the evaporator coilsof heat pumps as a function of the temperature, of the outside air, andof the refrigerant temperature.

Another object of this invention is to provide a simple temperature orpressure responsive control for refrigeration systems which willinitiate the defrost cycle upon an increase in temperature differentialbetween the air, and the temperature in the evaporator coil, and willterminate this cycle as a function of refrigerant temperature only.

A still further object is to provide a defrost control which above agiven evaporator temperature will not initiate a defrost cycle.

These and other objects of this invention which will become apparentfrom the following specification and claims are achieved by provision ofa novel control device which actuates an electric switch or othercontrol device which is hereshown as most optimumly arranged forregulating defrosting operations. The switch controls either theoperation of circulating fans, compressors, or valves depending on themode of defrosting employed. The novel control comprises a switch whichis arranged to initiate the defrosting cycle when a given amount ofrelative movement occurs between two temperature responsive members. Oneof these members responds to the temperature in the evaporator coil, andthe position of the other is dependent on the temperature in the ambientair passing over said evaporator. Reverse movement of the switch to cutout the defrost operation is accomplished solely as a function of theevaporator coil temperature responsive member. Thus the controlfunctions to regulate a given circuit for initiating a defrost operationin response to a temperature differential between the ambient air andthe evaporator coil. Above a given temperature the member responsive toambient temperatures is made ineffective, and defrosting is subject onlyto evaporator coil temperature.

The specific constructional features of this invention, and their modeof operation will be made most manifest and particularly pointed out inconjunction with the accompanying drawings, wherein FIGURE 1 representsa schematic cross-sectional view of a preferred control device embodyingthe hereindisclosed inventive concept;

FIGURE 2 is a cross-sectional view of an alternative embodiment of thisinvention;

FIGURE 3 is a graph in which the evaporator suction temperature at whichdefrosting is initiated is plotted against the outside temperature,indicating a suggested arrangement for adjustment of the control whenused in conjunction with defrosting a heat pump;

FIGURE 4 is a schematic view of a refrigeration system complete with thecontrol forming the invention; and

FIGURE 5 is a partial schematic view of the electrical circuit employedwith the system shown in FIGURE 4.

Referring now more particularly to the drawings, like numerals in thevarious figures will be taken to designate like parts.

As best seen in FIGURE 1, the thermally responsive control unit hereprovided comprises a primary bellows unit 11. The bellows unit 11 has arelatively rigid exterior cylindrical wall 12 which is sealed off at itstopmost portion, as viewed in the drawings by heat transmitting coverplate 13 and at its lowermost portion by bellows supporting closure 14.Closure 14 is formed with a central aperture, and is secured to abellows sleeve '15 interposed coaxially within cylindrical wall 12. Asseen in the drawing, the top portion 16 of the bellows sleeve is sealedoff so that the interior part of the bellows unit is formed with asealed annular chamber (between wall 12 and sleeve within which a givenreadily expansible fluid is provided. The fluid employed is any one of avariety of known expansible fluids, preferably of a volatile typeresponsive to temperature changes. A rod 17 is secured to the upperbellows wall 16, as viewed in the drawing. A spring 21 is mounted aboutrod 17 so as to bear against the upper wall of the bellows and against alower adjusting nut 22 forcing the bellows to a distended position. Thisadjusting nut 22 is formed with a threaded exterior surface engaging aninner tapped surface in the aperture provided in closure 14 of theassembly.

Push rod 17 is provided with a switch closing hub 18 engaging against aswitch 25 to move same. to the circuit closing position, as seen in thedrawing. A switch opening reversing hub 19 on rod 17 is formed with aflange 20 which may be threadably connected to the hub.

Switch is a conventional snap-acting switch of the over-center type andis arranged so as to be actuated by movement of rod 17. The snap-actingswitch is illustrated in its normally open position such that thecircuit is broken between terminals 26 and 27 of the circuit of whichthe switch forms a part.

A secondary bellows unit 30, as seen in the lowermost portion of FIGURE1, is formed, similar to the bellows unit 11, shown in the uppermostportion of the drawing and previously described. In the secondarybellows unit 30 a relatively rigid exterior cylindrical housing 31 isprovided. A conduit 32 leading to a temperature sensitive bulb 33 iscoupled to cylindrical housing 31 in any conventional manner, such forexample, as by means of coupling 34, as seen in FIGURE 1. Coaxiallymounted within the housing is a bellows sleeve 35 which serves to form asealed annular chamber within housing 31. As previously discussed, anytemperature responsive expansible fluid, preferably of a volatile type,is provided within the housing. Bellows sleeve 35 acts against spring36, which bears against movable cage 37. As seen in the drawing, cage 37has an opening admitting hub 19, and retaining flange 20. Cage 37 is ofa size such that hub 19 may move freely therein over a given distancefor a purpose to be made hereinafter more apparent.

Any forces transmitted by spring 36 to cage 37 are opposed by spring 38,acting on cover 39 of cage 37. A stop in the form of an annular flangedmember 40 is provided, recessed within housing 31 so that upper wall ofbellows 35 will, upon expansion of the volatile fluid within housing 31,strike against the cylindrical wall of stop member 40.

In the alternative embodiment illustrated in FIGURE 2 the bellows unitspreviously disclosed in the modification illustrated in FIGURE 1, aresubstituted by bi-metallic elements or thermal discs. The primarythermally responsive unit, here shown in the lowermost portion of thedrawing, is arranged so as to be encased within a housing fortransmitting the temperatures from the controlled unit to thebi-rnetallic disc 51. The housing 50 supports the bi-metallic thermalunit 51 and this thermal unit 51 is fastened to a push rod 52, which isarranged for movement within annular guides 53 and 54. An over-centerswitch 55, similar to switch 25 previously described, is arranged so asto be contacted by switch closing hub 60, and switch opening hub 61 onthe push rod 52. Switch 55 is arranged within a circuit containingconductors 56 and 57. The secondary thermally responsive member isretained within housing 58, here shown in the upper portion of thedrawing, and within this housing 58 thermal disc 59 is mounted. Thermaldisc 59 moves downwardly upon a drop in temperature, whereas thermaldisc 51 moves upwardly upon a drop in temperature. Housing 58 limitsupward movement of disc 59. As seen in the drawing, thermal disc 59 isnot secured to push rod 52, and hence restricts or constrains the motionof said rod, only when there is contact between rod 52 and disc 59.

Operation The novel control here provided functions to actuate a switch,positioned in any circuit which it is desired to regulate in response tovarying thermal conditions. As a general proposition, the control may beemployed in conjunction with a variety of circuits, but is contemplatedparticularly for use in refrigeration installations.

The mode of operation of the control is such that within a giventemperature range actuation of the switch is determined by a temperaturedifferential between two points, whereas reversing of the switch is afunction of the temperature at only one point. However, when thetemperature at one of these points rises above a given level, closing oropening of the switch is determined solely as a function of thetemperature at another point.

Optimum use can be made of the novel control, when employed inconjunction with a heat pump for regulating the defrost operation of therefrigeration system. As previously pointed out, where refrigerationunits are employed as heat pumps, defrosting of the evaporator coils ofthe system cannot be set to take place at a fixed temperature.

Empirical tests indicate that for heat pumps, optimum results obtainwhen the control is set to give a mode of operation as illustrated inFIGURE 3. At ambient temperatures above 40 F., defrosting takes placesolely as a function of coil temperatures, in this case 20 F. At outsidetemperatures below 40 F., defrosting is set to take place at atemperature 20 below the ambient temperature.

In the embodiment illustrated in FIGURE 1, the novel control structureis arranged to attain the above described mode of operation bypositioning primary bellows unit 11 adjacent the evaporator coils of arefrigeration unit employed as a heat pump. As illustrated in FIGURE 1,cover plate 13 of the control is strapped in some suitable fashion, asfor example by means of strap S to a return bend of coil C. i

It will be appreciated that coil C may be the outside coil in a heatpump having a motor-compressor unit 100 which, on the cooling cycle,feeds hot gaseous refrigerant to the coil C. The refrigerant isliquefied in coil C and passes through restriction 102 to an inside coil104 located within an enclosure to be supplied with conditioned air. Inthe inside coil refrigerant is converted to the gaseous state and flowsto the compressor to complete the cycle. In order to employ the heatpump to create a heating effect in the enclosure, a conventionalfour-way valve 103 for reversing flow of refrigerant through a portionof the system is provided. The valve is operable in response to asolenoid (not shown), the coil of which is coupled in the controlcircuit in series with switch 25, note FIGURE 5. When the circuit isclosed, by closing. heating-cooling switch 106, the valve is positionedso that refrigerant from the compressor flows to the inside coil,through the restriction and to the outside coil which functions as anevaporator and which may accumulate frost suflicient to necessitate thedefrost action that may be obtained by the unit forming the subject ofthis invention. Heat extracted from the ambient vaporizes therefrigerant which then passes through valve 103 to the compressor. Asuitable mounting bracket, as partially shown in diagrammatic FIGURE 4,is secured to the heat pump casing or any other convenient support sothat the connection referred to above may be accomplished. The bracketincludes extensions shown diagrammatically for mounting those parts ofthe thermal control requiring fixed support.

In view of the contact between cover 13, and coil C the surfacetemperature of coil C will be transmitted through plate 13 to theprimary bellows unit 11, thus, affecting the fluid contained within thebellows unit to expand or contract said fluid as a function ofevaporator coil temperature. I I,

Bulb 33 is positioned in the ambient air stream passing over theevaporator coils, and the volatile fluid contained within the fluidsystem comprised by secondary bellows unit 31, conduit 32 and bulb 33will expand or contract in response to the temperature conditions inthis ambient air stream. The amount of expansion or contraction of thefluid in secondary bellows unit 30 will determine the compression ofspring 36 which bears against cage 37 in opposition to the forcesexerted by spring 38 against flange 39 of the cage 37. It is thus seenthat the position of cage 37 is a function of the temperature of theambient air stream passing through the evaporator coils of the heatpump.

Over-center snap-switch 25 is positioned in the valve circuit of therefrigeration system to be controlled. Switch 25 is actuated by switchclosing hub 18, or switch opening reversing hub 19 positioned on pushrod 17. As is apparent, the motion of push rod 17 to force hubs 18, and19 against the switch 25 is dependent on the resultant of the forcesexerted by bellows 15, spring 21, bellows 35, spring 36, cage 37 andspring 38. The opening of the switch in the illustrated embodimentcauses the four way valve to move to the position for directing gaseousrefrigerant to the coil C. This is accomplished by deenergizing the coilcontrolling the solenoid regulating the valve.

Above a given temperature the fluid in bellows unit 30 has no eflect onthe position of cage 37, and hence push rod 17 since the bellow sleeve35 is restrained against stop 40.

With a conventional heat pump installation, in temperate climates atambient temperatures above 40, the formation of frost is dependent onlyon the surface temperature of the evaporator coil, and thus stop 40prevents the ambient temperature registered by bulb 33 from affectingthe operation of the switch. At these above 40 F. temperatures, switchoperation is purely a function of coil temperature as registered inprimary bellows unit 11, and hub 19 will move freely within cage 37 Withoutside temperatures below 40 the defrosting operation is made afunction of the temperature differential between the evaporator coilsurface and that of the ambient air stream. At these lower than 40temperatures push rod 17 will be constrained in its motion by cage 37engaging flange 20 of hub 19. The lower the temperature in the outsideair stream, the more force needed to move push rod 17, and the lower thetemperature necessary in the evaporator coil.

In FIGURE 1, the parts of the control are illustrated as functioningunder conditions of an outside ambient temperature below 40 F. Theswitch is shown as open, thus indicating a defrosting condition. Thecontrol is moving to close switch 25.

Secondary bellows unit 30 under the effect of a less than 40 F.temperature adjacent bulb 33 is distended (and not in contact with stop40) so as to retract cage 37. It will be noted that when hub 19 moves toopen switch 25, its motion under these conditions was restricted by cage37 engaging flange 20. However, when push rod 17 is moved by bellows 16to force hub 18 against switch 25 to close same to terminate the defrostoperation, cage 37 has no effect on the motion of rod 17. Hub 19 movesfreely in cage 37. Thus, termination of the defrosting operation ispurely a function of coil temperature; whereas, initiation of thedefrosting operation was dependent on the temperature differentialbetween the coil and ambient air stream.

Obviously, the spring pressures determining the push rod position may beadjusted to provide switch actuation at any desired temperaturedilferential. The described arrangement is set to function as seen inFIGURE 3 to pro. vide for a uniform cut out of the defrosting operationwhen the surface temperature of the coil reaches 40. The defrostingoperation is initiated at a 20 coil temperature when the outsidetemperature is at 40 or above, and at a correspondingly lowertemperature as the outside air temperature drops.

The control structure illustrated in FIGURE 2 may similarly be employedto regulate the defrost cycle of any refrigeration system by securingthe control to a return bend or other portion of evaporator coil C.Strap S is employed to secure contact between plate 50 and the coil thuspermitting heat transfer between the coil C and thermal disc 51. Coverplate 5 8 is exposed to the ambient air stream thus permitting thetemperature of the air stream to affect thermal disc 59.

The modification illustrated in FIGURE 2 is shown operating under anambient temperature condition of less than 40 F. Switch 55 is shownclosed, thus indicating a non-defrost condition. The position of thermaldisc 59, determined by the ambient temperature of the air stream passingover cover plate 58, is shown as affected by a temperature of less than40 F. As the ambient temperature drops, the disc will move downwardly tothe position shown. In the illustrated position disc 59 contacts pushrod 52 affecting the position thereof. Disc 59 is limited to movebetween plate 58 and guide 54.

Thermal disc 51 adjacent coil C is aifected by the temperature of thecoil, moving upwardly to the dotted line position as the coiltemperature drops. As seen, the forces exerted by the thermal discs 59and 51 act opposingly on push rod 5-2, which is secured to disc 51, butfree ,of disc 59.

When the temperature of coil C drops sufficiently to overcome the forcesexerted by disc 59 on the rod 52, the rod moves upwardly causing hub 61to engage switch 55 to break the circuit between conductors 56 and 57,thus initiating the defrost cycle.

When the temperature of coil C rises to 40", all frost will have beendissipated from coil C, and disc 51 will move downwardly as a result ofthe temperature increase. Push'rod 52 secured to disc 51 will movedownwardly with the disc and hub 60 will engage switch 55 in its dottedline position to close the circuit. It will be observed .that thecircuit closing action of hub 60 is purely a function of the motion ofdisc 51 in response to temperature conditions at coil C, since the rod52 is not affixed to disc 59 and thus rod 52 can leave disc 59 anddescend downwardly away from it.

At temperatures above 40 F., disc 59 assumes its upward limitingposition adjacent plate 58 and does not contact push rod 52. Thus allmotion of hubs 60 and 61 is dependent on the position of disc 51 and thecoil temperature.

It is thus seen that a simple relatively inexpensive control structurehas been provided for regulating the operation of a given mechanism inresponse to the temperature differential between two points. Morespecifically, a control structure has been provided particularly suitedfor control of the defrost operation of a refrigeration system employedas a heat pump, where it is desired to initiate the defrosting of theheat pump as a function of the temperature differential between theambient air stream and the evaporator coils under given ambienttemperature conditions (usually below 40 F.) while at higher ambienttemperatures, defrosting is purely a function of evaporator coiltemperature. Termination of the defrost cycle is made purely a functionof evaporator coil temperature.

The above disclosure has been given by way of illustration andelucidation, and not by way of limitation, and it is desired to protectall embodiments of the herein disclosed inventive concept within thescope of the appended claims.

I claim:

1. A defrost control for a frost accumulating coil in a refrigerationsystem including means for supplying heat to the coil to cause thedefrost action and a circuit regulating operation of the heat supplymeans, comprising a switch in said circuit; switch actuating means; afirst thermal unit, responsive to the temperature of the frostaccumulating coil, connected to said switch actuating means, said unitbeing operable upon a decrease in the temperature of the coil to exert aforce on said switch actuating mechanism in a direction to effectmovement of the switch to cause the defrost action; a second thermalunit responsive to the temperature of the air flowing over the coil;means operable to resist the action of the first unit to move saidactuator in a direction to effect movement of the switch, said meansbeing ineffective to resist reverse movement of the switch actuatorunder the influence of the first unit as it responds to an increase inthe coil temperature, said second thermal unit being operativelyassociated with said motion resisting means so as to regulate the extentof restraint whereby actuation of the switch to effect defrost occurs inresponse to a predetermined temperature differential within a givenoperating temperature range while reverse actuation of the switch may beeffective only in response to a coil temperature sufiicient toaccomplish complete defrost.

2. The invention set forth in claim 1 wherein said switch is of the overcenter action type.

3. The invention set forth in claim 1 wherein means are provided forlimiting the extent that said second thermal unit may vary therestraining action of said resisting means and thus establish a maximumcoil temperature at which defrost action may be accomplished.

4. In an air-to-air heat pump including an outdoor coil subject to frostaccumulation at relatively low ambient temperatures, defrost controlmeans comprising means regulating a supply of heat to the coil, acircuit governing the action of said heat supply means, a switchdisposed in said circuit, a rod for actuating said switch, means securedto the rod and operable upon a decrease in the temperature of the coilfor exerting a force on said rod for the purpose of moving it to engagesaid switch and effect defrost, means yieldably opposing movement of therod but not secured to the rod, and means operable in response to thetemperature of the air passing over the coil for regulating the extentof elfectiveness of the yieldable means opposing the rod movementwhereby the force necessary for switch movement to effect defrost isdeveloped in response to a predetermined temperature difference betweenthe air and coil over a given ambient temperature operating range, andthe switch movement, under the influence of the rod, for terminatingdefrost is in response to a coil temperature sufiicient to assurecomplete removal of frost.

References Cited in the file of this patent UNITED STATES PATENTS2,216,589 Grooms Oct. 1, 1940 2,666,298 Jones Jan. 19, 1954

